In recent years, a non-contact power supply device that supplies electric power in a non-contact state is widely used. The non-contact power supply device supplies electric power to a power reception device, such as a portable terminal and a tablet terminal, from a power transmission device in a non-contact state through electromagnetic coupling, such as an electromagnetic induction or a magnetic field resonance. The non-contact power supply device includes a power transmission circuit and a power transmission coil to transmit electric power. The power reception device includes a power reception coil for receiving electric power, a power reception circuit for using the received electric power and driving the power reception device, and a charging circuit for charging a secondary battery that is equipped with the power reception device.
In the non-contact power supply device, in order to supply the electric power to the power reception device from the power transmission device even in a case where devices are separated from each other more than 1 centimeter (cm) or 2 cm, a switching frequency having several megahertz (MHz) or more may be used in some cases. Such a switching frequency increases a quality (Q) value of a coil. For example, the switching frequency having 6.78 MHz or the switching frequency having 13.56 MHz is used. By increasing the Q value of the coil, it is possible to efficiently supply the electric power even when the power transmission coil and the power reception coil are far away from each other.
In the non-contact power supply device, it is possible to supply the electric power with respect to the power reception device with high efficiency, but it is necessary to suppress a radiation noise. In order to suppress the radiation noise, in some cases, an amplifier using a resonance, such as a class E amplifier that reduces a switching loss through zero-voltage switching (ZVS) or zero-current switching (ZCS), is used as the power transmission circuit of the non-contact power supply device. ZVS is a technique for switching a switching element such as a MOSFET in a zero-voltage state, and ZCS is a technique for switching a switching element such as a MOSFET in zero-current state. The ZVS and ZCS techniques are referred to as soft switching. The soft switching may reduce the switching loss, and realize the power transmission circuit with high efficiency.
In the non-contact power supply device in which the electric power may be supplied to the power reception device from the power transmission device even in a case where the power transmission coil and the power reception coil are separated from each other more than 1 cm or 2 cm, or more than several cm, the power reception coil is not tightly coupled with the power transmission coil. Thus, it is likely that electromagnetic waves are radiated to a space from the power transmission coil. In order to reduce the radiation noise radiated to the space from the power transmission coil, a low pass filter can be inserted into a previous stage of the power transmission coil to reduce harmonic components. However, since the amplifier in the non-contact power supply device performs the soft switching such as the ZVS or the ZCS by using an inductance (L) value of the power transmission coil and an inductance-capacitance (LC) resonance, insertion of an LC filter changes the resonance condition. Thus, the soft switching may operate incorrectly. As a result, there is a problem in that the circuit may operate incorrectly. In addition, in order for the resonance condition to remain unchanged in when the LC filter is inserted, a circuit constant is required to be reset. However, it can be difficult to adjust the circuit constant.
An induction heating device is a known device including the power transmission circuit that is used as a non-contact power supply device. The power transmission circuit in the non-contact power supply device of the induction heating device is configured such that the electric power is supplied to the power transmission coil from a high-frequency inverter that includes a switching element driven by a two-phase oscillator. The harmonic components radiated from the power transmission coil are reduced, and thus an L-type resonance circuit that includes an inductor and a capacitor is provided between the switching element and the power transmission coil. The L-type resonance circuit functions as the low pass filter. The switching frequency of the induction heating device is set as 80 kilohertz (KHz).
Since the resonance frequency of the L-type resonance circuit that functions as the low pass filter is required to be lower than the switching frequency, it is necessary that the inductors have relatively large values and be connected between the switching element and the power transmission coil in series. Since the large-value inductors are used, it is not possible to supply a sufficient electric current to the power transmission coil. As a result, the supplied power is also decreased.